1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) apparatus having balanced signal terminals provided at least for an input side and an output side and, more particularly, to a SAW apparatus having a balanced-to-unbalanced conversion function, and also relates to a communication unit including the above-described SAW apparatus.
2. Description of the Related Art
There has been significant technological progress in decreasing the size and the weight of communication apparatuses, such as cellular telephones. This progress has been achieved by reducing the number and the size of the individual components of cellular telephones. As the frequency used in cellular telephones increases to, for example, a GHz band, SAW apparatuses are important because the sizes of the individual components of the SAW apparatuses can be reduced.
Some SAW apparatuses require impedance matching devices, such as phase shifters, at the input/output terminals. Cited reference 1 (Japanese Unexamined Patent Application Publication No. 8-195645: Device-Mounting Package) discloses the following type of SAW apparatus. An impedance matching device is provided on a matching circuit substrate, and the matching circuit substrate is bonded to a SAW-device-mounted substrate. With this configuration, the resulting package is simplified.
Cited reference 2 (Japanese Unexamined Patent Application Publication No. 6-97761: Branching Filter and Method Therefor) discloses the following type of SAW apparatus. In a branching circuit, an impedance matching device and a phase matching device are embedded in a portion of a package other than a portion in which a SAW filter device is accommodated. With this arrangement, the size of the SAW apparatus is reduced while maintaining the isolation of the filter device.
To reduce the number of components, research has been actively conducted on SAW apparatuses provided with a balanced-to-unbalanced conversion function, i.e., a so-called xe2x80x9cbalun functionxe2x80x9d. Such SAW apparatuses are most widely used in Global System for Mobile communications (GSM) cellular telephones, which are used on a world-wide basis, particularly in Europe.
A balun is a circuit for matching a balanced line and an unbalanced line. More particularly, when a balanced line, such as a twin-lead type feeder, and an unbalanced line, such as a coaxial cable, are directly connected, a balanced current disadvantageously flows to cause the feeder itself to operate as an antenna. Accordingly, the balun circuit is provided to prevent the generation of a balanced current, thereby matching the balanced line and the unbalanced line.
Several patent applications concerning SAW apparatuses provided with the above-described balun function have been filed. As a SAW device used in a SAW apparatus provided with the balun function in which the input impedance and the output impedance are substantially the same, the configuration shown in FIG. 18 is commonly used.
In the SAW device shown in FIG. 18, on a piezoelectric substrate 100, a comb-like electrode (which is also referred to as an xe2x80x9cinterdigital electrodexe2x80x9d and is hereinafter referred to as an xe2x80x9cIDTxe2x80x9d) 101 is provided as the balanced portion. IDTs 102 and 103 are respectively provided on the left and right sides of the IDT 101 (in a SAW propagating direction) as the unbalanced portion. Reflectors 104 and 105 for improving transmission efficiency by reflecting the propagated SAW thereon are arranged such that they sandwich the IDTs 101, 102, and 103 therebetween. A SAW device having three IDTs arranged along a SAW propagating direction is referred to as a xe2x80x9c3 IDT-type extensionally-coupled-resonator-mode SAW devicexe2x80x9d.
In the above-described SAW device, reflectors 107 and 108 are arranged such that they sandwich an IDT 106 therebetween to define a SAW resonator. The SAW resonator is connected in series to the IDTs 102 and 103. Additionally, balanced signal terminals 109 and 110 connected to the IDT 101 and an unbalanced signal terminal 111 connected to the IDT 106 are also provided.
In a SAW apparatus provided with the above-described balun function, equal amplitude characteristics and 180xc2x0-inverted phase characteristics are required as transmission characteristics in a pass band between the unbalanced signal terminal 111 and each of the balanced signal terminals 109 and 110. The amplitude characteristics and the phase characteristics described above are referred to as the xe2x80x9camplitude balance degreexe2x80x9d and the xe2x80x9cphase balance degreexe2x80x9d, respectively.
The amplitude balance degree and the phase balance degree are defined as follows. When the above-described SAW device having a balun function is used as a three-port device, and when the unbalanced input terminal is a first port, and the balanced output terminals are a second port and a third port, the amplitude balance degree [A] is defined as A=[20log(S21)]xe2x88x92[(20log(S31)], and the phase balance degree is defined as [B-180] in which B is [∠S21xe2x88x92∠S31], where S21 is the transfer factor from the first port to the second port, and S31 is the transfer factor from the first port to the third port, and where the symbol [] represents an absolute value.
Ideally, in the pass band of a SAW apparatus, the amplitude balance degree is 0 dB, and the phase balance degree is 0 degrees. As one example of a SAW apparatus having a balun function, the SAW device shown in FIG. 18 is mounted on a package 200 shown in FIGS. 20 and 21.
On the reverse surface (external surface) of the package 200, as shown in FIG. 19, an external terminal 201, which is an unbalanced signal terminal, and external terminals 202 and 203, which are balanced signal terminals, are arranged along the peripheral portions of the reverse surface.
Within the package 200, as shown in FIG. 20, a die attach portion 204 is provided for holding the SAW device shown in FIG. 18 and for electrically connecting the SAW device to external devices. In wiring patterns of the die attach portion 204, the external terminal 201 and a wiring pattern 302, which are disposed adjacent to one another, are connected, the external terminal 202 and a wiring pattern 303, which are disposed adjacent to one another, are connected, and the external terminal 203 and a wiring pattern 304, which are disposed adjacent to one another, are connected.
In the known SAW apparatus, however, the following problems are presented.
In the package 200 shown in FIG. 19, the balanced-signal external terminals 202 and 203 cannot be symmetrically disposed with respect to the unbalanced-signal external terminal 201. Accordingly, the balance degrees between the balanced-signal external terminals 202 and 203 are decreased.
The reason for this is, for example, as follows. Because of the arrangement of the external terminals 201, 202, and 203 on the reverse surface of the package 200, the distance from the unbalanced-signal external terminal 201 to the balanced-signal external terminal 202 is different from the distance from the unbalanced-signal external terminal 201 to the balanced-signal external terminal 203. Accordingly, the bridge capacitance generated between the unbalanced-signal external terminal 201 and the balanced-signal external terminal 202 is different from the bridge capacitance between the unbalanced-signal external terminal 201 and the balanced-signal external terminal 203.
In the wiring patterns on the die attach portion 204 of the package 200, electrodes that are symmetrical with respect to the individual components cannot be provided, thereby decreasing balancing characteristics.
Additionally, in the SAW device shown in FIG. 18, the electrical polarity between the adjacent electrode fingers of the IDT 102 and the IDT 101 is different from that of the adjacent electrode fingers between the IDT 103 and the IDT 101. Accordingly, the balance degree of the SAW device itself is not sufficient, and thus, the balance degree of the SAW apparatus is not sufficient.
To solve these problems, the following SAW apparatus has been considered. As shown in FIG. 21, the wiring patterns are arranged by using strip lines 402 on the die attach portion 204 of the package 200, such that balanced-signal terminals 502 and 503 are disposed symmetrically with respect to an unbalanced-signal terminal 501, as shown in FIG. 22. In another configuration, an induction component defined by the strip lines 402 is provided to thereby improve the balance degree of the SAW device.
In the above-described configurations, however, when the strip lines 402 are provided on the die attach portion 204, an area for forming a bonding bump on the die attach portion 204 is decreased.
Accordingly, due to the reduced bump forming area on the SAW device, the flexibility of the layout on the SAW device is decreased. For example, the number of connecting portions between a grounding pattern of the die attach portion 204 and the package 200 is reduced, thereby weakening the grounding force. This adversely influences the electrical characteristics of the SAW device. Additionally, due to a limited number of bumps, the bonding strength is also decreased.
To overcome the above-described problems, according to one preferred embodiment of the present invention, a SAW apparatus is provided which includes a SAW device disposed on a piezoelectric substrate. The SAW device according to this preferred embodiment includes at least one interdigital electrode, and balanced signal terminals provided for at least an input side and an output side. A multi-layered retaining substrate includes external terminals for connecting the balanced signal terminals to an external device. The multi-layered retaining substrate is provided to retain the SAW device such that the interdigital electrode faces the surface of the multi-layered retaining substrate. An electrical circuit is arranged between the layers of the multi-layered retaining substrate such that the electrical circuit is located between the balanced signal terminals and the external terminals to increase balance degrees between the balanced signal terminals.
According to the above arrangement, since the SAW device includes at least one interdigital electrode and balanced signal terminals for at least the input side and the output side, a balun function is provided for the SAW device.
The retaining substrate is multi-layered, and an electrical circuit is provided between the layers of the retaining substrate. Accordingly, the flexibility to arrange the connecting and wiring patterns of the electrical circuit in the retaining substrate is greatly improved. Thus, electrical characteristics are uniform. For example, the bridge capacitance generated between the external terminal connected to the unbalanced signal terminal and the external terminal connected to one of the balanced signal terminals is approximately equal to the bridge capacitance generated between the external terminal connected to the unbalanced signal terminal and the external terminal connected to the other balanced signal terminal, thereby improving the balance degrees between the balanced signals.
Additionally, in the above-described configuration, by providing the electrical wiring pattern between the layers of the retaining substrate, connecting portions between the grounding wiring pattern on the retaining substrate and the grounding external terminal are increased such that a grounding force is strengthened. As a result, the attenuation in a frequency range other than the pass band is greatly improved.
In the aforementioned SAW apparatus, the external terminals are preferably arranged substantially symmetrically with respect to the center of a package by using the electrical circuit. Thus, electrical characteristics are uniform. For example, the bridge capacitance generated between the external terminal connected to the unbalanced signal terminal and the external terminal connected to one of the balanced signal terminals is approximately equal to the bridge capacitance generated between the external terminal connected to the unbalanced signal terminal and the external terminal connected to the other balanced signal terminal, thereby greatly improving the balance degrees between the balanced signals.
In the aforementioned SAW apparatus, the electrical circuit includes an adjusting portion provided for at least one of the balanced signal terminals of the SAW device to improve a signal propagation characteristic of the balanced signal terminal. The adjusting portion is defined by at least one of a delay line, a reactance component, and a resistance component, or other suitable component.
In the aforementioned SAW apparatus, the electrical circuit further preferably includes adjusting portions for both of the balanced signal terminals of the SAW device, and the adjusting degree of one of the adjusting portion for one of the balanced signal terminal is different from the adjusting degree of the other adjusting portion for the other balanced signal terminal.
With the above arrangement, a reactance component, a delay line, or a resistance component is added to one of the balanced signal terminals, or different reactance components, different delay lines, or different resistance components are added to the corresponding balanced signal terminals, thereby more reliably improving the balance degrees between the balanced signal terminals.
In the aforementioned SAW apparatus, the SAW device further includes a longitudinally-coupled resonator-mode SAW element having at least three interdigital electrodes. Generally, in extensionally-coupled resonator mode SAW elements, the electrical environment is different among the balanced signal terminals, and thus, the balance degrees between the balanced signal terminals substantially deteriorate. In the above-described configuration, however, due to the electrical circuit, the balance degrees are reliably maintained.
In the aforementioned SAW apparatus, the SAW device is constructed such that an electrical neutral point is not provided between the balanced signal terminals. Generally, in a SAW device provided without an electrical neutral point between the balanced signal terminals, it is difficult to arrange the external terminals symmetrically with respect to the center of the retaining substrate. In the above-described configuration, however, because of the multi-layered retaining substrate and the electrical circuit, the balance degrees between the balanced signal terminals are greatly improved.
According to another preferred embodiment of the present invention, a communication apparatus includes at least one of the SAW apparatuses according to preferred embodiments of the present invention described above. With this configuration, by providing the composite SAW apparatus which exhibits excellent transmission characteristics, the number of components is decreased, and accordingly, the size of the overall communication apparatus is reduced. Additionally, the transmission characteristics of the communication apparatus are greatly improved.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments thereof with reference to the attached drawings.